Multiple structure cube corner article and method of manufacture

ABSTRACT

A retroreflective cube corner article which is a replica of a directly machined substrate comprises a plurality of single cube corner elements which are machined in the substrate. Each cube corner element is bounded by at least one groove from each of three sets of parallel grooves in the substrate so that the article comprises a plurality of different geometric structures.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 08/326,690,filed Oct. 20, 1994, now U.S. Pat. No. 5,557,836, which is acontinuation-in-part of U.S. patent application Ser. No. 08/140,638,filed Oct. 20, 1993, now abandoned.

FIELD OF INVENTION

This invention relates to retroreflective articles having prismaticretroreflective elements.

BACKGROUND

Many types of retroreflective elements are known, including prismaticdesigns incorporating one or more structures commonly known as cubecorners. Retroreflective sheeting which employs cube corner typereflecting elements is well known. Cube corner reflecting elements aretrihedral structures which have three approximately mutuallyperpendicular lateral faces meeting in a single corner. Light rays aretypically reflected at the cube faces due to either total internalreflection or reflective coatings. The manufacture of directly machinedarrays comprising retroreflective cube corner elements has manyinefficiencies and limitations. Total light return and percent activeaperture are adversely affected by these limitations, and overallproduction costs versus performance are often higher relative to the newclass of articles and methods of manufacture taught below. The multiplestructure arrays of this invention permit excellent manufacturingflexibility and production of cube corner element designs which arehighly tailorable to particular needs.

SUMMARY OF INVENTION

The invention comprises a method of manufacturing a cube corner articlecomprising the steps of providing a machinable substrate materialsuitable for forming reflective surfaces, and creating a plurality ofgeometric structures including individual cube corner elements in thesubstrate. The step of creating the cube corner elements comprisesdirectly machining at least three sets of parallel grooves in thesubstrate so that the intersections of the grooves within two groovesets are not coincident with at least one groove in a third groove set.The separation between the intersections of the grooves within twogroove sets and at least one groove in a third groove set is greaterthan about 0.01 millimeters.

The invention also comprises a retroreflective cube corner article whichis a replica of a directly machined substrate in which a plurality ofgeometric structures including cube corner elements are machined in thesubstrate. Each cube corner element is a single cube corner elementwhich is bounded by at least one groove from each of three sets ofparallel grooves in the substrate. The intersections of the grooveswithin two groove sets are not coincident with at least one groove in athird groove set.

The invention also comprises a retroreflective cube corner article whichis a replica of a directly machined substrate in which a plurality ofgeometric structures including cube corner elements forming an array aremachined in the substrate. Each cube corner element is bounded by atleast one groove from each of three sets of parallel grooves. Thearticle exhibits at least two different active aperture sizes at zeroentrance angle.

The invention comprises a retroreflective cube corner article which is areplica of a directly machine substrate in which a plurality ofgeometric structures including cube corner elements formed in an arrayare machined in the substrate. Each cube corner element is a single cubecorner element which is bounded by at least one groove from each ofthree sets of parallel grooves in the substrate. The array exhibits aplurality of different active aperture shapes.

The invention comprises a retroreflective cube corner article which is areplica of a directly machined substrate in which a plurality ofgeometric structures including cube corner elements are machined in thesubstrate. Each cube corner element is bounded by at least one groovefrom each of three sets of parallel grooves in the substrate so that thearticle comprises a plurality of different geometric structures.

The invention comprises a retroreflective cube corner article which is areplica of a directly machined substrate in which a plurality ofgeometric structures including cube corner elements are machined in thesubstrate. Each cube corner element is bounded by at least one groovefrom each of three sets of parallel grooves in the substrate so that, inplan view, at least one of the structures has more than three sides andless than six sides.

The invention comprises a method of manufacturing an article having aplurality of cube corner elements formed by directly machining threesets of grooves into a machineable substrate in any order. The methodcomprises the steps of directly machining a first groove set of parallelgrooves along a first path in the substrate; directly machining a secondgroove set of parallel grooves along a second path in the substrate tocreate a plurality of rhombus shaped partial cube sub-elements; anddirectly machining a third groove set comprising at least one additionalgroove along a third path in the substrate, so that a plurality ofdifferent optically retroreflective geometric structures is formed inthe article.

The invention comprises a retroreflective cube corner article which is areplica of a directly machined substrate in which at least twogeometrically different matched pairs of cube corner elements aremachined in the substrate by grooves from each of three sets of parallelgrooves in the substrate.

The invention comprises a retroreflective cube corner article which is areplica of a directly machined substrate in which a plurality ofgeometric structures including cube corner elements are machined in thesubstrate. The shape of the active aperture of at least one cube cornerelement is determined at least in part by an edge of the cube corner notcoincident with the base.

The invention comprises a retroreflective cube corner article which is areplica of a directly machined substrate in which a plurality of groovesin groove sets are machined in the substrate to form structuresincluding cube corner elements. The article exhibits asymmetric entranceangularity when rotated about an axis within the plane of the substrate.

The invention comprises a retroreflective cube corner element compositesheeting comprising a plurality of zones of retroreflective cube cornerelements. Each zone comprises a replica of a directly machined substratein which a plurality of cube corner elements are machined. The compositesheeting comprises at least one zone comprising geometric structuresincluding retroreflective cube corner elements which exhibits asymmetricentrance angularity when rotated about an axis within the plane of thesubstrate.

The invention comprises a retroreflective cube corner element compositesheeting comprising a plurality of zones of retroreflective cube cornerelements. Each zone comprises a replica of a directly machined substratein which a plurality of geometric structures including individual cubecorner elements are machined. Each individual cube corner element isbounded by at least one groove from each of three sets of parallelgrooves in the substrate. The grooves are arranged so that a pluralityof different optically retroreflective geometric structures is formed inat least one zone of the sheeting.

The invention comprises a retroreflective cube corner element compositesheeting comprising a plurality of zones of retroreflective cube cornerelements. Each zone comprises a replica of a directly machined substratein which a plurality of geometric structures including individual cubecorner elements are machined. Each individual cube corner element isbounded by at least one groove from each of three sets of parallelgrooves. The intersections of the grooves within two groove sets are notcoincident with at least one groove in a third set in at least one zoneof the sheeting.

The invention comprises a retroreflective cube corner element compositesheeting composite sheeting comprising a plurality of zones ofretroreflective cube corner elements. Each zone comprises a replica of adirectly machined substrate in which a plurality of geometric structuresincluding cube corner elements are machined. Each cube corner element isan individual cube corner element. At least one matched pair of cubecorner elements with no coincident base edges is machined in thesubstrate in at least one zone of the sheeting.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a plan view of a directly machinable substrate having twogroove sets machined into the substrate to form partial cube shapes.

FIG. 2 is a section view of the substrate taken along line 2--2 in FIG.1.

FIG. 3 is a section view of the substrate taken along line 3--3 in FIG.1.

FIG. 4 is an elevation view of a machine tool used to form the groovesshown in FIG. 1.

FIG. 5 is a section view of the substrate taken along line 5--5 in FIG.1.

FIG. 6 is a plan view of a portion of a directly machined three grooveset array having matched pairs of retroreflective elements.

FIG. 7 is a section view of the array taken along line 7--7 in FIG. 6showing individual non-canted cube corner element symmetry axesperpendicular to a base plane.

FIG. 8 is a plan view of a portion of a directly machined three grooveset array having a plurality of canted cube corner elements.

FIG. 9 is a section view of the array taken along line 9--9 in FIG. 8,including the symmetry axes of the cubes.

FIG. 10 is a plan view of the zero entrance angle active apertures ofthe array shown in FIGS. 6 and 7.

FIG. 11 is a section view of a portion of an array having individualcube corner elements configured in an extreme backward cant.

FIG. 12 is a plan view of a portion of the array of FIG. 11.

FIG. 13 is a section view a portion of an array similar to that shown inFIGS. 11 and 12 modified by reducing the length of each cube cornerelement and by eliminating one cube vertical optical face.

FIG. 14 is a plan view of a portion of the array of FIG. 13.

FIG. 15 is a plan view of a portion of a directly machined substrate.

FIG. 16 is an elevation view of a half angle tool used to machine thesubstrate shown in FIG. 15.

FIG. 17 is a section view of the substrate taken along line 17--17 inFIG. 15.

FIG. 18 is a section view of the substrate taken along line 18--18 inFIG. 15.

FIG. 19 is a section view of the substrate taken along line 19--19 inFIG. 15.

FIG. 20 is a plan view of a directly machined array having threenon-parallel non-mutually intersecting sets of grooves.

FIG. 21 is a section view of the array taken along line 21--21 in FIG.20.

FIG. 22 is a plan view of the active apertures of a portion of amultiple structure retroreflective cube corner element array depicted inFIG. 20.

FIG. 23 is a plan view of a portion of a directly machined multiplegeometric structure array having cube corner elements with cantedsymmetry axes, variable groove centering, and variable cube types.

FIG. 24 is a perspective view of variably shaped active apertures of anarray of the invention.

FIG. 25 is a plan view of a multiple structure cube corner element arrayformed from primary and secondary grooves intersecting with includedangles of 82°, 82°, and 16°.

FIG. 26 is a schematic view of active apertures of the array shown inFIG. 25 at a 60° entrance angle.

FIG. 27 is a plan view of a directly machined multiple structure arrayincluding a plurality of cube corner elements formed from primary andsecondary grooves intersecting with included angles 74°, 74°, and 32°.

FIG. 28 is a schematic view of active apertures of the array shown inFIG. 27 at a 60° entrance angle.

FIG. 29 is a plan view of a portion of a directly machined three grooveset multiple structure cube corner element array having cube cornerelements with canted symmetry axes, and variable groove centering.

FIG. 30 is a schematic view of active apertures of the array shown inFIG. 29 at a 0° entrance angle.

FIG. 31 is a schematic view of active apertures of the array shown inFIG. 29 at a 30° entrance angle.

FIG. 32 is a graph depicting the percent active aperture as a functionof entrance angle for the multiple structure array shown in FIG. 29.

FIG. 33 is a plan view of a portion of a directly machined multiplestructure cube corner element array having variable spacing betweengrooves.

FIG. 34 is a plan view of a portion of a directly machined multiplestructure cube array having variable spacing between grooves and inwhich at least one of the cube corner elements is not a part of amatched pair.

FIG. 35 is a plan view of a portion of a composite array comprisingseveral zones of multiple structure arrays.

FIG. 36 is a section view of one embodiment of a multiple structurearray having truncated surfaces.

FIG. 37 is a section view of one embodiment of a multiple structurearray having a separation surface.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The manufacture of retroreflective cube corner element arrays isaccomplished using molds made by different techniques, including thoseknown as pin bundling and direct machining. Molds manufactured using pinbundling are made by assembling together individual pins which each havean end portion shaped with features of a cube-corner retroreflectiveelement. U.S. Pat. No. 3,926,402 (Heenan et al) and U.S. Pat. No.3,632,695 (Howell) are examples of pin bundling.

The direct machining technique, also known generally as ruling,comprises cutting portions of a substrate to create a pattern of grooveswhich intersect to form cube corner elements. The grooved substrate isreferred to as a master from which a series of impressions, i.e.replicas, may be formed. In some instances, the master is useful as aretroreflective article, however replicas, including multi-generationalreplicas, are more commonly used as the retroreflective article. Directmachining is an excellent method for manufacturing master molds forsmall micro-cube arrays. Small micro-cube arrays are particularlybeneficial for producing thin replica arrays with improved flexibility,such as continuous rolled goods for sheeting purposes. Micro-cube arraysare also more conducive to continuous process manufacturing. The processof manufacturing large arrays is also relatively easier using directmachining methods rather than other techniques. One example of directmachining is shown in U.S. Pat. No. 4,588,258 (Hoopman).

FIG. 1 illustrates a method by which directly machined masters ofconventional cube arrays are manufactured. A directly machinablesubstrate 20 receives a plurality of parallel grooves 23, arranged intwo non-parallel sets. Grooves through directly machinable substrate 20are formed by a machine tool with two opposing cutting surfaces forcutting cube corner optical faces. Examples of shaping, ruling, andmilling techniques suitable for forming directly machined grooves arediscussed in U.S. Pat. No. 3,712,706 (Stamm). The two sets of groove 23produce the partial cube shapes 39 depicted in the sectional views ofFIG. 2 and FIG. 3. Machine tool 26, such as that shown in FIG. 4, istypically mounted on a post 35 and has an optical face cutting surface29 on each side of a tool central axis 32.

In FIGS. 1-4, partial cube shapes 39 are shown as rhombus shapedstructures formed in substrate 20. At least two grooves 23 in bothnon-parallel groove sets are required to produce shapes 39. At least onethird groove 41, as shown in sectional view dashed lines in FIG. 5, isrequired to produce conventional cube corner elements. Portions of aconventional cube array 42 after completion of the three groove sets areshown in FIGS. 6 and 7. Both sides of all grooves 23, 41 form cubecorner element optical surfaces in array 42. An equilateral triangle isformed at the base of each cube corner reflecting element 44, 45. All ofthe cube corner element shapes have three sides when viewed in planview. The grooves 23 and 41 mutually intersect at representativelocations 43. Another example of this grooving is shown in U.S. Pat. No.3,712,706 (Stamm). U.S. Pat. Nos. 4,202,600 (Burke et al) and 4,243,618(Van Arnam) also disclose, and incorporate by reference, the triangularbased corner reflecting elements or prisms shown in Stamm. The Burke etal patent discloses tiling of these prisms in multiple differentlyoriented zones to produce an appearance of uniform brightness to the eyewhen viewed at a high angle of incidence from at least a minimumexpected viewing distance.

Conventional retroreflective cube corner element arrays are derived froma single type of matched pairs, i.e. geometrically congruent cube cornerretroreflecting elements rotated 180°. These elements are also typicallythe same height above a common reference plane and share a coincidentbase edge. One example of this single matched pair derivation is shownin FIG. 6 with matched shaded pair of cube corner retroreflectingelements 44, 45 having a coincident base edge along groove 41. Otherexamples of this fundamental matched pair concept relating toconventional cube arrays is shown in U.S. Pat. No. 3,712,706 (Stamm),U.S. Pat. No. 4,588,258 (Hoopman), U.S. Pat. No. 1,591,572 (Stimson) andU.S. Pat. No. 2,310,790 (Jungersen). U.S. Pat. No. 5,122,902 (Benson)discloses another example of matched pairs of cube cornerretroreflecting elements having coincident base edges, although thesemay be positioned adjacent and opposite to each other along a separationsurface.

Another type of matched pair of cube corner elements is disclosed inGerman patent reference DE 42 42 264 (Gubela) in which a structure isformed having a micro-double triad and two single traids within arhombic body. The structure is formed in a work piece using turningangles of 60 degrees and grinding directions which do not cross eachother at one point, resulting in only two of the directions having acommon point of intersection.

The above examples of cube corner element retroreflective arrayscomprise non-canted cubes which have individual symmetry axes 46, 47that are perpendicular to a base plane 48, as shown in FIG. 7. Thesymmetry axis is a central or optical axis which is a trisector of theinternal or dihedral angles defined by the faces of the element.However, in some practical applications it is advantageous to cant ortilt the symmetry axes of the matched pair of cube cornerretroreflective elements to an orientation which is not perpendicular tothe base plane. The resulting canted cube-corner elements combine toproduce an array which retroreflects over a wide range of entranceangles. This is taught in U.S. Pat. No. 4,588,258 (Hoopman), and isshown in FIGS. 8 and 9. The Hoopman structure is manufactured with threesets of parallel V-shaped grooves 49, 50, 51 that mutually intersect toform a single type of geometrically congruent matched pairs of cantedcube corner elements 53, 54 in array 55. All of the cube corner elementshapes have three sides when viewed in plan view. Both sides of allgrooves 49, 50, 51 form cube corner element optical surfaces in array55.

FIG. 9 illustrates the symmetry axis 57 for cube corner element 53, andthe symmetry axis 58 for cube corner element 54. The symmetry axes areeach tilted at angle .o slashed. with respect to a line 60 that liesnormal to a base plane 63, or the front surface, of the element. Thebase plane is usually coplanar or parallel with the front surface of asheeting comprising the cube corner element array. Cube corner elements53, 54 are geometrically congruent, exhibit symmetric opticalretroreflective performance with respect to entrance angle when rotatedabout an axis within the plane of the substrate, and have symmetry axeswhich are not parallel to each other. Entrance angle is commonly definedas the angle formed between the light ray entering the front surface andline 60.

Canting may be in either a forward or backward direction. The Hoopmanpatent includes disclosure of a structure having an amount of cant up to13° for a refractive index of 1.5. Hoopman also discloses a cube with acant of 9.736°. This geometry represents the maximum forward cant ofcubes in a conventional array before the grooving tool damages cubeoptical surfaces. The damage normally occurs during formation of a thirdgroove when the tool removes edge portions of adjacent elements. Forexample, as shown in FIG. 8, for forward cants beyond 9.736°, the cubeedge 65 is formed by the first two grooves 49, 50 and is removed byforming the primary groove 51. U.S. Pat. No. 2,310,790 (Jungersen)discloses a structure which is canted in a direction opposite that shownin the Hoopman patent.

For these conventional arrays, optical performance is convenientlydefined by the percent of the surface area that is actuallyretroreflective, i.e. which comprises an effective area or activeaperture. The percent active aperture varies as a function of the amountof canting, refractive index, and the entrance angle. For example,shaded areas 68 of FIG. 10 represent the active apertures of theindividual cube corner retroreflective elements in array 42. Thehexagonal percent active aperture of this equilateral 60°-60°-60° baseangle geometry array at a zero entrance angle is about 67 percent, whichis the maximum possible for a conventional three groove array. All thehexagonal active apertures have the same size and shape in this example.

At non-zero entrance angles, conventional arrays display, at most, twodifferent aperture shapes of roughly similar size. These result from thesingle type of geometrically congruent matched pairs of conventionalcube corner elements. Canted conventional cube corner arrays exhibitsimilar trends, although the shape of the aperture is affected by thedegree of canting.

As discussed in U.S. Pat. No. 5,171,624 (Walter), diffraction from theactive apertures in nearly orthogonal conventional cube corner arraystends to produce undesirable variations in the energy pattern ordivergence profile of the retroreflected light. This results from allthe active apertures being roughly the same size in conventional arraysand therefore exhibiting roughly the same degree of diffraction duringretroreflection.

The active apertures of conventional arrays are determined by the baseedges of the cubes. For example, the six sides of the hexagonalapertures 68 of FIG. 10 are determined by the three base edges and theimage of these edges reflected through the cube peak. The three baseedges and their image generally determine the aperture shape forconventional canted and uncanted arrays at any entrance angle.

Some conventional cube corner arrays are manufactured with additionaloptical limitations, perhaps resulting from canting or other designfeatures, to provide very specific performance under certaincircumstances. One example of this is the structure disclosed in U.S.Pat. No. 4,349,598 (White). FIGS. 11 and 12 schematically depict, inside and plan views respectively, White's extreme backward cantassociated with one geometric limit of a conventional cube design. Inthis design, cube structure 73 is derived from a matched pair of cubecorner elements 74, 75 with symmetry axes 77, 78. Cube corner elements74, 75 are each canted in a backward direction to the point that each ofthe base triangles is eliminated, resulting in two vertical opticalfaces 79, 80. This occurs when the cube peaks 81, 82 are directly abovethe base edges 83, 84 and the base triangles have merged to form arectangle. Only two groove sets are required, using tools with opposingcutting surfaces, to create this cube structure in a substrate. Onegroove set has a 90° V-shaped cut 85 and the other groove set has arectangular cut shaped as a channel 86. Both sides of all grooves 85, 86form cube corner element optical surfaces in array 73. In the Whitedesign, the pair of cube corner reflecting elements are specificallyarranged to provide a high active aperture at large entrance angles. Theaperture shapes for the White design are bounded by the base. Also, thestructure disclosed by White has four sides in plan view.

A further modification to the conventional cube corner arrays and to theWhite design is disclosed in U.S. Pat. No. 4,895,428 (Nelson et al). Thecube structure 87 disclosed by Nelson et al, shown in the side view ofFIG. 13 and the plan view of FIG. 14, is derived by reducing the lengthof the White element 73 and by eliminating one of the cube verticaloptical faces 79, 80. Like the White design, manufacture of the Nelsonet al structure also requires only two groove sets 88, 89. Both sides ofall the grooves 88 form cube corner element optical surfaces in array87. Nelson must also have at least one vertical retroreflective face.This is accomplished by replacing the tool for cutting the Whiterectangular channel with an offset tool. The Nelson et al tool forms anon-retroreflective surface 90, using a tool relief surface, and avertical retroreflective surface 92 using the tool vertical sidewall.The aperture shapes for the Nelson design are bounded by the base. Also,the structure disclosed by Nelson has four sides in plan view. U.S. Pat.No. 4,938,563 (Nelson et al) further modifies the White design by theaddition of, inter alia, canted bisector elements.

Conventional cube corner retroreflective element designs includestructural and optical limitations which are overcome by use of themultiple structure cube corner retroreflective element structures andmethods of manufacture described below. Use of this new class ofmultiple structure retroreflective cube corner element structures andmanufacturing methods permits diverse cube corner element shaping. Forexample, cubes in a single array may be readily manufactured with raiseddiscontinuous geometric structures having different heights or differentshapes. Use of multiple structure methods and structures also permitsmanufacture of cube arrays which have highly tailorable opticalperformance. For example, at many entrance angles, including at zeroentrance angle, multiple structure arrays outperform conventional arraysby exhibiting higher percent active apertures or by providing improveddivergence profiles, or both. Multiple structure manufacturingtechniques may also produce enhanced optical performance resulting fromclosely spaced intermixed cubes with different active aperture shapesand sizes. This presents more uniform appearances of multiple structurearrays over a wide range of viewing distances under both day and nightobservation conditions. Multiple structure arrays may also be based onmore than one type of matched pair of cube corner elements. Matchedpairs may have base edges which are non-coincident or which have only asingle point of intersection which is common. These advantages ofmultiple structure cube corner elements enhance the usefulness ofarticles having these elements. Such articles include, for example,traffic control materials, retroreflective vehicle markings,photoelectric sensors, directional reflectors, and reflective garmentsfor human or animal use.

Multiple structure cube corner element arrays may be of unitary orcomposite, i.e. tiled, construction, and may be formed using tools witheither one or both sides configured for cutting retroreflective opticalsurfaces. Manufacture of multiple structure cube corner element masterarrays, as well as multi-generational replicas, results in diverse andhighly adaptable optical performance and cost efficiencies. These andother advantages are described more fully below.

A substrate suitable for forming retroreflective surfaces according tothis invention may comprise any material suitable for forming directlymachined grooves or groove sets. Suitable materials should machinecleanly without burr formation, exhibit low ductility and lowgraininess, and maintain dimensional accuracy after groove formation. Avariety of materials such as machinable plastics or metals may beutilized. Suitable plastics comprise thermoplastic or thermosetmaterials such as acrylics or other materials. Suitable metals includealuminum, brass, nickel, and copper. Preferred metals includenon-ferrous metals. Preferred machining materials should also minimizewear of the cutting tool during formation of the grooves.

FIG. 15 discloses one method by which directly machined masters ofmultiple geometric structure cube corner element arrays aremanufactured. A directly machined substrate 100 receives a plurality ofparallel grooves arranged in two non-parallel sets, which may havevariable spacing between grooves. Grooves may be formed using eithersingle or multiple passes of a machine tool through substrate 100. Eachgroove is preferably formed by a machine tool which has only one sideconfigured for cutting a retroreflective non-vertical optical surfaceand which is maintained in an approximately constant orientationrelative to the substrate during the formation of each groove. Eachgroove forms the side surfaces of geometric structures which may includeindividual cube corner optical or non-optical elements.

A more detailed description of a method of manufacturing a multiplestructure cube corner element array is to directly machine a firstgroove set 104 of parallel grooves 106 cut into substrate 100 along afirst path. A second groove set 107 of parallel grooves 108 is thendirectly machined along a second path in substrate 100. The machining ofthe first and second groove sets, also referred to as the two secondarygrooves or secondary groove sets, creates a plurality of rhombus ordiamond shaped partial cube sub-elements 109, depicted in shadedhighlight in one instance for ease of recognition. Each partial cubesub-element comprises two orthogonal optical faces 110, as shown inFIGS. 15, 17, 18 and 19. Preferably, only one side of grooves 106 and108 form the orthogonal faces 110 on partial cube sub-element 109. Thesecondary grooves intersect at locations 114. Multiple structure arraysmay be compared to conventional arrays at this point of manufacture bycomparing analogous views of FIGS. 1 and 15, 2 and 19, 3 and 17, and 5and 18. After formation of the secondary grooves, a third or primarygroove set, which may contain as few as one groove, is cut along a thirdpath in substrate 100. In FIG. 18, a representative primary groove 116,which in this example mutually intersects the secondary grooves 106 and108, is shown in dotted lines. A more detailed discussion of suchprimary groove(s) is found below in relation to groove set 128 andgroove(s) 130 depicted in the array embodiment of FIG. 20.

Each of the secondary grooves 106, 108 are preferably formed using anovel half angle tool 118, shown in one embodiment in FIG. 16. Therelief angle X may be any angle, although a preferred range of angles isbetween 0° and 30°. In FIGS. 15, and 17-23 relief angle X is 0°. Thetool side angle Y shown in FIG. 16 is non-zero and preferably specifiedto create orthogonal or nearly orthogonal cube optical surfaces. Afterformation of the secondary grooves, a third or primary groove set 128,which may contain as few as one groove 130, is preferably cut using apass along a third path in substrate 100. The addition of a plurality ofparallel primary grooves 130 is shown in FIGS. 20 and 21. Third grooveset 128 is cut through partial cube sub-elements so that non-cantedindividual cube corner elements 134, 135, 136, with cube peaks 137, 138,139 are formed by the intersections of the primary groove(s) with theorthogonal faces of the partial cube sub-elements. Cube corner elements134, 135, 136 comprise multiple geometric structures which, in planview, have either three or six sides in array 141.

The invention also comprises a method of manufacturing a retroreflectivecube corner article which is a replica of a directly machined substratein which a plurality of geometric structures including cube cornerelements are formed in the substrate. In this embodiment of theinvention, each cube corner element is bounded by at least one groovefrom each of three sets of parallel grooves in the substrate. It isrecognized that grooves or groove sets in a method of forming cubecorner elements according to this invention may comprise a differentscope and meaning from grooves or groove sets which bound or form a cubecorner element in known articles. For example, in known articles,multiple passes of a machine tool may be required to form a singlegroove.

Other embodiments of this method include creation of an article, orreplicas of the article, which further modify the shape of theretroreflected light pattern. These embodiments comprise at least onegroove side angle in at least one set of grooves which differs from theangle necessary to produce an orthogonal intersection with other facesof elements defined by the groove sides. Similarly, at least one set ofgrooves may comprise a repeating pattern of at least two groove sideangles that differ from one another. Shapes of grooving tools, or othertechniques, may create cube corner elements in which at least asignificant portion of at least one cube corner element optical face onat least some of the cubes are arcuate. The arcuate face may be concaveor convex. The arcuate face, which was initially formed by one of thegrooves in one of the groove sets, is flat in a direction substantiallyparallel to said groove. The arcuate face may be cylindrical, with theaxis of the cylinder parallel to said groove, or may have a varyingradius of curvature in a direction perpendicular to said groove.

FIG. 20 further discloses a multiple structure cube array 141 in whichprimary grooves 130 do not pass through the secondary grooves 106, 108at the mutual intersection locations 114 of the secondary grooves.Primary grooves 130 are equally spaced and centered on secondary grooveintersection locations 114. Array 141 presents yet another novel featureof multiple structure cube corner technology. In particular, a method isdisclosed for manufacturing a cube corner article by directly machiningthree non-parallel non-mutually intersecting sets of grooves.Preferably, these sets intersect at included angles less than 90°. It isrecognized that certain machining imprecisions may create minor,unintentional separation between grooves at intersections. However, thisinvention involves intentional and substantial separation. For example,a separation distance between the intersections of the grooves withintwo groove sets with at least one groove in a third groove set which isgreater than about 0.01 millimeters would likely provide the advantagesof this feature. However, the precise minimum separation distance isdependent on the specific tooling, substrate, process controls, and thedesired optical performance sought.

Non-mutually intersecting groove sets create multiple geometricstructures including cube corner elements with different active aperturesizes and shapes. Arrays may even be formed with cube corners created bya combination of mutually and non-mutually intersecting groove sets. Theposition of the groove sets is controlled to produce maximum total lightreturn over a desired range of entrance angles. Also the distancebetween grooves in at least one groove set might not be equal to thedistance between grooves in at least another of the groove sets. It isalso possible to machine at least one set of parallel grooves into asubstrate in a repeating fashion with the set comprising a distancebetween grooves which is optionally variable at each machining of theset. Also, a portion of any one of the grooves may be machined to adepth that is different from at least one other groove depth.

FIG. 21 illustrates the multiple cube surfaces which are formed duringdirect machining of a groove in substrate 100. FIG. 21 shows that theplurality of optical surfaces and cube peaks 137, 138, 139 are createdat different heights above a common reference plane 154.

FIG. 22 is a plan view of a portion of multiple structureretroreflective cube corner element array 141 with shaded areas 155,156, 157 representing three different active apertures, intermixed andarranged in close proximity and corresponding to cube types 134, 135,and 136. A conventional non-canted cube corner element array with anequilateral base triangle, at 0° entrance angle, provides a maximum ofonly about 67 percent active aperture. However, a multiple structurecube corner element array similar to that shown in FIG. 23 may have apercent active aperture greater than about 70 percent and possibly ashigh as about 92 percent at 0° entrance angle.

FIGS. 23 and 24 illustrate a multiple structure array 165 with thesymmetry axis canted forward by 21.78°. Each of the primary grooves 167has a 4° relief angle, and each of the secondary grooves 169, 170 has a20° relief angle. The secondary groove intersection locations 171 aredesigned with a spacing distance D₁. Primary grooves 167 are equallyspaced, also with the distance D₁, and are positioned at 0.155D₁ fromeach adjacent intersection location 171. This pattern is repeated inother partial cube sub-elements. The array of FIG. 23 comprises aplurality of different, i.e., not congruently shaped, geometricstructures including three different cube types depicted by numerals172, 173, and 174 respectively. The different geometric structures inFIG. 23 have bases with either three or five sides when observed in planview.

FIG. 24 shows the multiple differently sized and shaped active apertures184, 185, 186, intermixed and arranged in close proximity, andcorresponding to the three cube types numbered 172, 173, and 174 at a60° entrance angle and a refractive index of 1.59. Total percent activeaperture for array 165 is roughly 59 percent under these conditions.Aperture 186 is an example of an active aperture which is determined inpart by an edge of the cube corner not coincident with the base. Thisdesign is useful in applications requiring high brightness at highentrance angles, for example, in pavement markers, roadway dividers,barriers, and similar uses.

The invention permits numerous combinations of structures previouslyunknown and not possible within the art of retroreflective cube cornerelement design and manufacture, and in particular within the art ofdirectly machined retroreflective cube corner element design andmanufacture. FIG. 25 discloses, in plan view, multiple structure cubecorner element array 191 formed from primary and secondary groovesintersecting with included angles 82°, 82°, and 16°. Primary grooves areequally spaced through array 191, with some of the primary groovesmutually intersecting the secondary grooves at locations 194. In thisembodiment, the primary grooves 195 have a 30° relief angle, and thesecondary grooves 196, 197 have a 3° relief angle. The array of FIG. 25comprises a plurality of different geometric structures including sevendifferent cube types depicted by numerals 198, 199, 200, 201, 202, 203,and 204, respectively. The different geometric structures in FIG. 25have either three or four sides when viewed in plan view. Numerousdifferent retroreflective cube corner elements are created which werenot possible using previous manufacturing technologies.

When a light ray enters array 191 at a 60° entrance angle, and using arefractive index of 1.59, the array demonstrates an exceptional 63percent active aperture as schematically shown in FIG. 26. This percentactive aperture represents the combined optical performance of multipledifferently sized and shaped apertures 212, 213, 214, 215, 216, 217, and218, intermixed and arranged in close proximity, and corresponding tothe different types of retroreflective cube corner elements 198, 199,200, 201, 202, 203, and 204. Array 191 is also useful in applicationsrequiring high brightness at high entrance angles such as pavement orchannel markers, roadway dividers, barriers, and similar uses.

FIGS. 27 and 28 illustrate a multiple structure array 305 comprising aplurality of cube corner elements each formed from primary and secondarygrooves intersecting with included angles 74°, 74°, and 32°. Each of theprimary grooves 308 has a 30° relief angle and each of the secondarygrooves 309, 310 has a 3° relief angle. The secondary grooveintersection locations 313 are designed with a spacing D₂. Three primarygrooves are positioned in the partial cube sub-element with varyingspacing at 0.20D₂, 0.55D₂, and 0.83D₂ from the secondary grooveintersections 313. This pattern is repeated in other partial cubesub-elements.

In the array of FIG. 27, there are six different cube types depicted bynumerals 316, 317, 318, 319, 320, and 321. Trihedrons 325, 326 areexamples of structures which are not retroreflective because the threefaces are not orthogonal. FIG. 28 shows, for a 60° entrance angle and arefractive index of 1.59, the six active apertures 329, 330, 331, 332,333, and 334, intermixed and arranged in close proximity, which areassociated with cube types numbered 316 through 321, respectively.Percent active aperture for this array is approximately 63 percent, asshown in FIG. 28. The active aperture shapes in this design have roughlyequal dimensions both parallel and perpendicular to the primary grooveseven at a 60° entrance angle. These roughly circular aperture shapesproduce light return patterns which are relatively circular and notsignificantly distorted due to diffraction. In contrast, conventionalarrays specifically designed for high entrance angle high brightnessapplications exhibit highly elongated aperture shapes whichsignificantly distort light return patterns. The multiple structurearray 305 is particularly useful in applications requiring highbrightness at high entrance angles, such as pavement or channel markers,roadway dividers, barriers, and similar uses.

As discussed above, many limiting cases of conventional cube cornerelement design are surpassed through use of multiple structure methodsof manufacture. In some multiple structure designs, cube surfaces havingsome conventional cube geometries may occur as part of a plurality ofcube types in a single array. However, the normal limits of conventionalcube shapes and performances are not similarly bounded using multiplestructure methods and structures.

FIG. 29 discloses another method by which directly machined masters ofmultiple structure cube corner element arrays may be manufactured. Amultiple structure array 400 with the symmetry axis canted forward by9.74° is formed using three sets of parallel grooves in a directlymachined substrate. Each groove is preferably formed by a full anglemachine tool which has two sides configured for cutting aretroreflective optical surface and which is maintained in anapproximately constant orientation relative to the substrate during theformation of each groove. The full angle tools used to cut this multiplestructure array are similar to those used to cut a conventional cantedcube array such as described by Hoopman and shown above in FIG. 8.However, proper relative placement of the grooves in this multiplestructure array results in improved and highly adaptable opticalperformance, improved physical characteristics, and cost efficiencies.These and other advantages are described more fully below.

The secondary grooves 405, 406 intersect at locations 407 which aredesigned with a spacing distance D₃. Primary grooves 410 do not mutuallyintersect the secondary grooves at locations 407, are equally spacedwith the distance D₃, and are positioned at 0.25D₃ from each adjacentintersection location 407. In this embodiment, this pattern is repeatedin other partial cube sub-elements. The array of FIG. 29 comprises aplurality of different geometric structures including two differenttypes of matched pairs of cube corner elements. The combination of cubeelements 415 and 416 is representative of one type of matched pair whichis not coincident along any base edge and share only a common basevertex. The combination of cube elements 420 and 421 is representativeof a second type of geometrically different matched pair of cubes whichshare a coincident base edge.

The different geometric structures in FIG. 29 have either three or fivesides when viewed in plan view. The plurality of structures includingcube corner elements have different heights above a common referenceplane. The intersections 407 of the secondary grooves 405, 406 are notcoincident with any portion of the primary grooves 410 in this example,so the three sets of grooves do not mutually intersect.

FIG. 30 shows the two differently sized active apertures 440 and 441,intermixed and arranged in close proximity, at a 0° entrance angle.Active aperture 440 corresponds with cubes 415 and 416 while activeapertures 441 corresponds with cubes 420 and 421. The active aperturesfor both cubes in each matched pair of cube corner elements areidentical at 0° entrance angle. Total percent active aperture for thismultiple structure array is approximately 62.5% at 0° entrance angle,which substantially exceeds the 50 percent active aperture possible forconventional arrays canted to the same degree such as shown in FIG. 8.

At non-zero entrance angle, the four cube corner elements in the twomatched pairs produce four differently sized and shaped activeapertures. An example for a 30° entrance angle and a refractive index of1.59 is presented in FIG. 31, where active apertures 450, 451, 452, and453 correspond to multiple structure cube corner elements 415, 416, 420,and 421, respectively. The shape of aperture 452 is determined in partby edges 455 of the cube corner structure which are not coincident withthe base. Total percent active aperture for this multiple structurearray is roughly 48 percent at 30° entrance angle, which also exceedsthe roughly 45 percent active aperture possible for conventional arrayscanted to the same degree such as that shown in FIG. 8.

FIG. 32 shows percent active aperture as a function of entrance angle incurve 460 for multiple structure array 400 shown in FIG. 29 and in curve461 for a conventional Hoopman array, both of which are canted 9.74°.Both arrays exhibit symmetric entrance angularity when rotated about anaxis in the plane of the sheeting. The multiple structure array exhibitshigher percent active aperture at entrance angles up to roughly 35° fora refractive index of 1.59. At high entrance angles, the Hoopman arrayexhibits higher percent active aperture. However, multiple structurearray 400 continues to retroreflect a significant amount of the incidentlight, even at very high entrance angles. This combination of relativelyhigh light return at up to 35° entrance angles combined with adequatelight return at very high entrance angles surpasses the performance ofconventional canted and non-canted cube corner arrays. It is furtherrecognized that a multiple structure array may be made which combinesboth mutually and not mutually intersecting grooves.

Variable groove spacing within any groove set may be used to producemultiple structure cube arrays with additional beneficial features. Onesuch array 480 is presented in FIG. 33, where secondary grooveintersections 407 are again designed with spacing D₃, similar to FIG.29. However, the spacing of the primary grooves 470 relative to thesecondary groove intersections 407 is varied in a repeating patternthroughout array 480. This multiple structure array with the symmetryaxis again canted forward by 9.74° is formed with three sets of parallelgrooves using full angle tools in a directly machined substrate. Sixdifferent types of matched pairs of cube corner elements are formed inarray 480, with several of the pairs shown in shaded lines in FIG. 33.Each of the pairs has a different geometric structure. The six matchedpairs comprising elements 415 and 416, 420 and 421, 488 and 489, 490 and491, 492 and 493, and 494 and 495 may not share a coincident base edge,or even have a base vertex in common. For example, matched pairs ofelements 488 and 489, 490 and 491, and 494 and 495 are completelyseparated within array 480. A wide range of aperture sizes and shapeswill result in this array, with a corresponding improvement in theuniformity of the return energy pattern or divergence profile of theretroreflected light due to diffraction. Proper placement of grooves canbe utilized advantageously during design to provide optimum productperformance for a given application. The use of multiple matched pairswill again result in an array which exhibits asymmetric entranceangularity when rotated about an axis in the plane of the sheeting.

FIG. 34 discloses another array 500 having variable groove spacing in atleast one of the groove sets. However, the spacing in FIG. 34 producesan array in which at least one of the cube corner elements is not a partof a matched pair. For example, cube corner elements 415 and 416 as wellas 420 and 421 form matched pairs in array 500 while cube cornerelements 488, 490, 492, and 494, each shown in shaded lines, are nolonger part of matched pairs in FIG. 34. Elements 489, 491, 493, and 495from FIG. 33, which were matched elements, no longer exist in array 500.This array will not exhibit symmetric entrance angularity when rotatedabout an axis in the plane of the sheeting. FIG. 33 and FIG. 34 furtherdisclose examples of retroreflective cube corner articles which may bereplicas of a directly machined substrate. These arrays have a pluralityof groove sets which form optical surfaces, and at least one of thegrooves forms cube corner element optical surfaces comprising lateralfaces of geometric structures on only a portion of at least one side ofthe selected groove(s).

Conventional Hoopman arrays cannot be canted past the 9.74° limitwithout the mutually intersecting primary grooving tool removing theedges formed by the secondary grooves on adjacent cube elements.Multiple structure arrays such as those in FIGS. 29, 33, and 34 areformed using three sets of parallel grooves which do not necessarilymutually intersect. Therefore, cantings past the conventional limit maybe beneficially used in multiple structure arrays without damagingadjacent cube elements and impairing optical performance.

Multiple structure geometries are particularly beneficial for use inapplications requiring retroreflective sheeting having substantial totallight return, such as traffic control materials, retroreflective vehicleor approach markings, photo-electric sensors, signs, internallyilluminated retroreflective articles, reflective garments, andretroreflective markings. The enhanced optical performance and designflexibility resulting from multiple structure techniques and conceptsrelates directly to improved product performance and marketingadvantage.

Total light return for retroreflective sheeting is derived from theproduct of percent active aperture and retroreflected light rayintensity. For some combinations of cube geometries, entrance angles,and refractive index, significant reductions in ray intensity may resultin relatively poor total light return even though percent activeaperture is relatively high. One example is retroreflective cube cornerelement arrays which rely on total internal reflection of theretroreflected light rays. Ray intensity is substantially reduced if thecritical angle for total internal reflection is exceeded at one of thecube faces. Metallized or other reflective coatings on a portion of anarray may be utilized advantageously in such situations. A portion, inthis context, may comprise all or part of an array.

Composite tiling is the technique for combining zones of cube cornerelements having different orientations. This is used, for example, withconventional arrays to provide sheeting with a uniform appearance athigh angles of incidence regardless of orientation. In another example,composite tiling may be introduced to provide symmetric opticalperformance with respect to changes in entrance angle using arrays whichindividually exhibit asymmetric entrance angularity, as well as tomodify the optical performance of arrays comprising non-triangular basedcube corner prisms.

Referring to FIG. 35, composite array 552 comprises several zones ofasymmetric arrays 165. Composite arrays may comprise several zones ofdifferent arrays including at least one zone comprising multiplestructure arrays. Adjacent zones of multiple structure arrays may havedifferent size and relative orientation. The size of the zones should beseIected according to the requirements of particular applications. Forexample, traffic control applications may require zones which aresufficiently small that they are not visually resolvable by the unaidedhuman eye at the minimum expected viewing distance. This provides acomposite array with a uniform appearance. Alternatively, channelmarking or directional reflector applications may require zones whichare sufficiently large that they can be easily resolved by the unaidedhuman eye at maximum required viewing distance.

FIG. 36 is a side section view of one embodiment of the presentinvention. This view shows part of a multiple structure array 564 whichis similar to array 141 shown in FIG. 21, although this embodiment ofthe invention may also be used with other array configurations. FIG. 36further illustrates the advantages of multiple structure manufacturingmethods in providing geometric structures at different heights above acommon reference plane and utilizing varying depth of groove duringmachining. For example, at least a portion of groove 576 is machined toa depth into the substrate which is different from the depth of groove575. The multiple structures in array 564 may comprise individualretroreflective cube corner elements 568, 569, non-retroreflectivepyramids, frustums, posts 582, or other structures positioned abovecommon reference plane 574.

Cube peaks 571, 572, or other features machined from the originalsubstrate, may also be truncated for specialized effect or use.Truncation may be accomplished by various techniques, including, forexample, controlling depth of cut of the grooves, or further removal ofsubstrate material after formation of the primary and secondary grooves.

Retroreflective directly machined cube corner articles are oftendesigned to receive a sealing film which is applied to theretroreflective article in order to maintain a low refractive indexmaterial, such as air, next to the retroreflective elements for improvedperformance. In conventional arrays this medium is often placed indirect contact with the cube corner elements in ways which degrade totallight return. However, using multiple structure constructions, a sealingmedium 580 may be placed on the highest surface 583 of the array withoutcontacting and degrading the optical properties of lower retroreflectivecube corner elements. The highest surface may comprise cube cornerelements, non-retroreflective pyramids, frustums, posts, or otherstructures. In FIG. 36, the highest surface 583 has been truncated.Although slight height variations may result from slight non-uniformityof groove positions or included angle of cube corner elements due tomachining tolerances or intentional inducement of non-orthogonality,these variations are not analogous to the variations disclosed andtaught in this invention. For arrays using a sealing medium, thetruncated surfaces may be used both to hold the medium above the cubecorner elements as well as to increase the light transmissivity of thesheeting. Light transmissivity of the sheeting may be increased throughuse of a transparent or partially transparent sealing medium.

FIG. 37 is a section view of another embodiment of the presentinvention. This view shows part of a multiple structure array 585similar to a portion of array 564 in FIG. 36 but including the use of aseparation surface 588. The lateral faces 592, 593 of geometricstructures 595, 596 form the boundary edges 599, 600 for the separationsurface. The lateral faces may include cube corner element opticalsurfaces as well as non-optical surfaces on cube corner and othergeometric structures. The separation surface 588 may have flat or curvedportions when viewed in cross section.

Separation surfaces may be advantageously utilized to increase lighttransmission or transparency in sheeting, including flexible sheeting,utilizing multiple structure retroreflective cube corner element arrays.For example, this is particularly useful in internally illuminatedretroreflective articles such as signs or automotive signal lightreflectors, which are normally manufactured using injection molding. Inthe embodiment shown in FIG. 37, separation surfaces are shown incombination with truncated surfaces of highest surfaces 583, althougheither feature may be utilized independently. Separation surface 588 maybe formed using a machining tool with a flat or curved tip, or byfurther removal of material from a replica of the multiple structurecube corner element array master.

Suitable materials for retroreflective articles or sheeting of thisinvention are preferably transparent materials which are dimensionallystable, durable, weatherable, and easily replicated into the desiredconfiguration. Examples of suitable materials include glass; acrylics,which have an index of refraction of about 1.5, such as Plexiglas brandresin manufactured by Rohm and Haas Company; polycarbonates, which havean index of refraction of about 1.59; reactive materials such as taughtin U.S. Pat. Nos. 4,576,850, 4,582,885, and 4,668,558; polyethylenebased ionomers, such as those marketed under the brand name of SURLYN byE. I. Dupont de Nemours and Co., Inc.; polyesters, polyurethanes; andcellulose acetate butyrates. Polycarbonates are particularly suitablebecause of their toughness and relatively higher refractive index, whichgenerally contributes to improved retroreflective performance over awider range of entrance angles. These materials may also include dyes,colorants, pigments, UV stabilizers, or other additives. Transparency ofthe materials ensures that the separation or truncated surfaces willtransmit light through those portions of the article or sheeting.

The incorporation of truncated and/or separation surfaces does noteliminate the retroreflectivity of the article, but rather it rendersthe entire article partially transparent. In some applications requiringpartially transparent materials, low indices of refraction of thearticle will improve the range of light transmitted through the article.In these applications, the increased transmission range of acrylics(refractive index of about 1.5) is desirable.

In fully retroreflective articles, materials having high indices ofrefraction are preferred. In these applications, materials such aspolycarbonates, with refractive indices of about 1.59, are used toincrease the difference between the indices of the material and air,thus increasing retroreflection. Polycarbonates are also generallypreferred for their temperature stability and impact resistance.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention.

What is claimed:
 1. A retroreflective sheeting, comprising a substratehaving a base surface and a structured surface opposite said basesurface, said structured surface including:a first groove set comprisingat least two parallel grooves; a second groove set comprising at leasttwo parallel grooves, said second groove set intersecting said firstgroove set at a plurality of intersection locations; and a third grooveset comprising at least two parallel grooves, at least a first groove ofsaid third groove set intersecting said first groove set and said secondgroove set at a point displaced from said intersection locations todefine an array of cube corner elements bounded by at least one groovefrom each of said groove sets; wherein: each cube corner element in saidarray is defined by three mutually perpendicular faces that intersect ata cube corner peak.
 2. Retroreflective sheeting according to claim 1,wherein:said at least one groove of said third groove set intersectssaid first and second groove sets at least 0.01 millimeters from saidintersection locations of said first and second groove sets. 3.Retroreflective sheeting according to claim 1, wherein:said arraycomprises a plurality of non-congruently shaped cube corner elements. 4.Retroreflective sheeting according to claim 1, wherein:the symmetry axisof a plurality of cube corner elements in said array are canted withrespect to an axis perpendicular to said base surface. 5.Retroreflective sheeting according to claim 1, wherein:the cube cornerelements of said sheeting exhibit at least two different activeapertures in response to light incident on said base surface at a zerodegree entrance angle.
 6. Retroreflective sheeting according to claim 5,wherein:the cube corner elements of said sheeting exhibit a plurality ofdifferent active apertures in response to light incident on said basesurface at a zero degree entrance angle.
 7. Retroreflective sheetingaccording to claim 1, wherein:at least one of the cube corner elementscomprises a lateral face having more than three sides. 8.Retroreflective sheeting according to claim 1, wherein:at least twogeometrically different matched pairs of cube corner elements aremachined in the substrate by grooves from each of three sets of parallelgrooves in the substrate.
 9. Retroreflective sheeting according to claim8, wherein:at least one matched pair of cube corner elements has nocoincident base edges between each cube corner element. 10.Retroreflective sheeting according to claim 1, wherein:at least one cubeface is arcuate over a significant portion of the cube surface. 11.Retroreflective sheeting according to claim 10, wherein:the shape of thearcuate surface is substantially cylindrical, so that the axis of thecylinder is approximately parallel to the groove which bounds thearcuate surface.
 12. Retroreflective sheeting according to claim 1,wherein:at least one groove side angle in at least one set of groovesdiffers from the angle that would produce an orthogonal intersectionwith other surfaces of cube corner elements.
 13. Retroreflectivesheeting according to claim 1, wherein:said sheeting exhibits asymmetricentrance angularity.
 14. Retroreflective sheeting according to claim 1,wherein:the grooves in any one set are not equidistant. 15.Retroreflective sheeting according to claim 1, wherein:substratecomprises a substantially optically transparent material suitable foruse in retroreflective articles.
 16. Retroreflective sheeting accordingto claim 1, wherein:a portion of said sheeting is opticallytransmissive.
 17. Retroreflective sheeting according to claim 1, furthercomprising:a sealing medium disposed adjacent said structured surface.18. A method of manufacturing a cube corner article, comprising thesteps of:providing a machinable substrate; machining in said substrate afirst groove set comprising at least two parallel grooves; machining insaid substrate a second groove set comprising at least two parallelgrooves, said second groove set intersecting said first groove set at aplurality of intersection locations; and machining in said substrate athird groove set comprising at least two parallel grooves, at least afirst groove of said third groove set intersecting said first groove setand said second groove set at a point displaced from said intersectionlocations to define an array of cube corner elements bounded by at leastone groove from each of said groove sets; such that each cube cornerelement in said array is defined by three mutually perpendicular facesthat intersect at a cube corner peak.
 19. The method of claim 18,wherein:said third groove set is machined such-that said array comprisesa plurality of non-congruently shaped cube corner elements.
 20. Themethod of claim 18, wherein:at least one groove set is provided with arelief angle, said relief angle measuring at least 3 degrees.
 21. Themethod of claim 18, wherein:at least a portion of one groove is machinedto a depth into the substrate that is different from the depth of atleast one other groove.
 22. The method of claim 18, further comprisingthe step of:removing a portion of at least one cube corner element insaid array to form a geometric structure suitable for supporting asealing medium.
 23. A cube corner article manufactured by the method ofclaim
 18. 24. A cube corner article which is a replica of the article ofclaim
 23. 25. A retroreflective sheeting comprising a substrate having abase surface and a structured surface opposite said base surface, saidstructured surface including:a first groove set consisting of aplurality of parallel grooves; a second groove set consisting of aplurality of parallel grooves that intersect grooves of the first grooveset at a plurality of intersection locations; and a third groove setconsisting of a plurality of parallel grooves that intersect said firstgroove set and said second groove set at points displaced from saidintersection locations to define an array of cube corner elementsbounded by at least one groove from each of said three groove sets;wherein each cube corner element in said array is defined by threemutually perpendicular faces that intersect at a cube corner peak. 26.Retroreflective sheeting according to claim 25, wherein:grooves of saidthird groove set intersect said first and second groove sets at pointsdisplaced from said intersection locations by a distance of at least0.01 millimeters.
 27. Retroreflective sheeting according to claim 25,wherein:said array comprises a plurality of non-congruently shaped cubecorner elements.
 28. Retroreflective sheeting according to claim 25,wherein:the cube corner elements of said sheeting exhibit a plurality ofdifferent active apertures in response to light incident on said basesurface at a zero degree entrance angle.
 29. Retroreflective sheetingaccording to claim 25, wherein:grooves of the first groove set areseparated by a distance d₁, grooves of the second groove set areseparated by a distance d₂, and grooves of the third groove set areseparated by a distance d₃, and wherein the distance d₃ is differentfrom the distances d₁ and d₂.
 30. Retroreflective sheeting according toclaim 25, wherein:the distance d₃ between grooves of the third grooveset is variable.
 31. Retroreflective sheeting according to claim 25,wherein:at least one groove is formed using an asymmetric tool.